Aaron S. Evans

NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the Hubble Space Telescope NICMOS camera for a sample of nine luminous [LIGs: LIR(8?1000 ?m) ? 1011 L?] and 15 ultraluminous (ULIGS: LIR ? 1012 L?) infrared galaxies. The sample includes representative systems classified as warm (f25 ?m/f60 ?m > 0.2) and cold (f25 ?m/f60 ?m ? 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as H II (i.e., starburst), QSO, Seyfert, and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters. The colors and the luminosities of the observed clusters are consistent with them being young (107?108 yr), formed as a result of galactic interactions, and having masses much greater than those of Galactic globular clusters. In NGC 6090 and VV 114, they are preferentially situated along the area of overlap of the two galactic disks. With the exception of IR 17208-0018, all of the ULIGs have at least one compact (2.2??m FWHM ? 200 pc) nucleus. Analysis of the near-infrared colors (i.e., m1.1?1.6 vs. m1.6?2.2) derived from 11 diameter apertures suggests that the warm galaxies have near-infrared colors consistent with QSO+hot dust emission and the cold galaxies, as a group, have near-infrared colors consistent with reddened starlight. In addition, the cold ULIG UGC 5101 (and possibly three others) have near-infrared colors suggesting additional active galactic nucleus?like near-infrared components in their nuclei. In a 2 kpc diameter aperture measurement, the global colors of all of the cold galaxies except UGC 5101 are consistent with starlight with a few magnitudes of visual extinction. The general dichotomy of the near-infrared properties of the warm and the cold galaxies are further supported by the light distributions: seven of the eight warm galaxies have unresolved nuclear emission that contributes significantly (i.e., ?30%?40%) to the total near-infrared luminosity. The smooth, more extended light observed in all of the galaxies is most likely composed of giant and supergiant stars, but evidence at longer wavelengths suggests that these stars contribute little to the high 8?1000 ?m luminosity of these galaxies. Finally, light profiles of nine of the 24 systems were fitted well by an r1/4 law (and not so well by an exponential disk profile). Whether these star systems eventually become massive central bulges or giant elliptical galaxies will depend on how efficiently the present ISM is converted into stars.

Molecular Gas in the z = 2.565 Submillimeter Galaxy SMM J14011+0252

We report the detection of CO (3?2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5?1010 h?275 M?, after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter-selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas, which is consistent with the inferred high rates of star formation (103 M? yr-1). These two systems appear to be associated with merger events which may evolve into present-day luminous elliptical galaxies.

HIGH-RESOLUTION OPTICAL/NEAR-INFRARED IMAGING OF COOL ULTRALUMINOUS INFRARED GALAXIES

We present high spatial resolution BIHK@-band imaging of a sample of ultralu- (FWHM B 0A.3¨0A.8) minous infrared galaxies ULIGs) with ii cool ˇˇ mid-infrared colors (L ir ( 1012 L _ ;( f 25 km /f 60 km \ 0.2), which select against active galactic nucleuslike (AGN-like) systems, a complementary sample to the ii warm ˇˇ ULIGs of Surace and coworkers. We —nd that all of the cool ULIGs are either advanced mergers or are premergers with evidence for still-separate nuclei with separations greater than 600 pc. Extended tidal features such as tails and loops as well as clustered star formation are observed in most systems. This extended tidal structure suggests a common progenitor geometry for most of the ULIGs: a plunging disk collision where the disks are highly inclined with respect to each other. The underlying host galaxies have H-band luminosities of 1¨2.5L *, very similar to that found in the warm ULIGs. The nuclear regions of these galaxies have morphologies and colors characteristic of a recent burst of star formation mixed with hot dust and mildly extinguished by mag; only in one case (IRAS A V \ 2¨5 22491(1808) is there evidence for a compact emission region with colors similar to those ofan extin- guished QSO. Most of the observed star-forming knots appear to have very young (10 Myr) ages based on their optical/near-infrared colors. These star-forming knots are insufficiently luminous to typically provide more than 10% of the high bolometric luminosity of the systems. Subject headings: galaxies: activegalaxies: interactionsgalaxies: star clusters ¨ galaxies: starburstinfrared: galaxies

Fe K emission in the ultraluminous infrared galaxy Arp 220

Prominent Fe Kα line emission is detected in the XMM‐Newton spectrum of the ultraluminous infrared galaxy Arp 220. The centroid of the line is found at an energy of 6.7 keV and the equivalent width of the line is EW ∼ 1.9 keV (at 3.5σ significance). A few other spectral features are found at various degrees of significance in the lower energy range on a hard 2.5‐ 10 keV continuum (� ∼ 1). The large EW of the Fe K line poses a problem with interpreting the hard X-ray emission as integrated X-ray binary emission. A thermal emission spectrum with a temperature of kT ∼ 7k eV modified by absorption of N H � 3 × 10 22 cm −2 , can describe the 2.5‐10 keV continuum shape and the Fe K emission. A hot bubble that is shocked internally in a starburst region would have a similar temperature and gives a good explanation for the observed X-ray properties with a high star formation rate. An ensemble of radio supernovae in a dense environment, as suggested from VLBI imaging, could be another possibility, if such powerful supernovae are produced continuously at a high rate. However, the apparent lack of emission from X-ray binaries is incompatible with the high supernova rate (∼2 SNe yr −1 ) required by both interpretations. Highly photoionized, low-density gas illuminated by a hidden Compton-thick active galactic nucleus is a possible alternative for the hard X-ray emission, which can be tested by examining whether radiative recombination continua from highly ionized Ca and Fe are present in better quality data from a forthcoming observation. Ke yw ords: galaxies: individual: Arp 220 ‐ X-rays: galaxies.

Molecular Gas in 3C 293: The First Detection of CO Emission and Absorption in a Fanaroff-Riley Type II Radio Galaxy

The —rst detection of CO emission in a FanaroU and Riley type II (i.e., edge-brightened radio lobe morphology) radio galaxy is presented. Multiwavelength (0.36¨2.17 km) imaging of 3C 293 shows it to be a disk galaxy with an optical jet or tidal tail extending toward what appears to be a companion galaxy 28 kpc away via a low surface brightness envelope. The molecular gas appears to be distributed in an asymmetric disk rotating around an unresolved continuum source, which is presumably emission from the active galactic nucleus (AGN). A narrow km s~1) absorption feature is also (*v abs D 60 observed in the CO spectrum and is coincident with the continuum source. Assuming the standard CO conversion factor, the molecular gas mass is calculated to be 1.5 ) 1010 several times the (H 2 ) M _ , molecular gas mass of the Milky Way. The high concentration of molecular gas within the central 3 kpc of 3C 293, combined with the multiwavelength morphological peculiarities, supports the idea that the radio activity has been triggered by a gas-rich galaxy-galaxy interaction or merger event. Subject headings: galaxies: activegalaxies: individual (3C 293) ¨ galaxies: ISM ¨ infrared: galaxiesISM: moleculesradio lines: galaxies

Molecular gas and nuclear activity in ultraluminous infrared galaxies with double nuclei

High-resolution CO(1 ! 0) observations of five ultraluminous infrared galaxies (ULIGs: LIR[8 1000µm] � 10 12 L⊙) with double nuclei are analyzed. These sources constitute a complete subset of local ULIGs expected to be in an intermediate stage of merging and selected with projected nuclear separations of 2. ′′ 0 5. 4 (3–5 kpc) so they could be resolved with the Owens Valley Radio Observatory Millimeter Array. The observed pairs include two mergers with cool far-infrared colors (25µm to 60µm flux density ratio f25µm/f60µm < 0.2) from the Infrared Astronomical Satellite (IRAS) Bright Galaxy Sample (IRAS 12112+0305 and IRAS 14348-1447) and three mergers with warm infrared dust temperatures (f25µm/f60µm � 0.2) selected from the IRAS Warm Galaxy Sample (IRAS 08572+3915, IRAS 13451+1232 = PKS 1345+12, and IRAS 13536+1836 = Mrk 463). These ULIGs are further distinguished by the presence of pairs of active nuclei; among the ten nuclei, nine have Seyfert or LINER classifications and one is unclassified. Molecular gas is detected only on the redder, more radio-luminous nucleus of the warm objects, whereas both nuclei of the cool ULIGs are detected in CO. The inferred molecular gas masses for the detected nuclei are 0.1 1.2×10 10 M⊙, and the undetected nuclei have molecular gas masses at least 1.2–2.8 times less than that of their CO-luminous companions. Upper limits on the extent of the CO emitting regions of each detected nucleus range from 2–4 kpc, which is about 3-6 times smaller than the average effective CO diameter of nearby spiral galaxies. This is strong evidence that the high concentration of molecular gas is the result of tidal dissipation in ongoing mergers. There is no correlation between the optical emission-line classification of the nuclei (i.e., Seyfert, LINER, or H II) and the presence of detectable molecular gas; however, there is a clear indication that the relative amount of molecular gas increases with the relative level of activity as measured via radio power and optical/near-infrared emission-line strength. Star formation rates are estimated to be in the range � 30 290 M⊙ year −1 nucleus −1 by making assumption that the radio and infrared emission arise from supernovae and dust heating by massive stars, respectively; corresponding gas consumption timescales are 1 7 × 10 7 years. The nuclei detected in CO are extremely red at near-infrared wavelengths, suggestive of much dustier environments than in the companions undetected in CO. Column density estimates are NH2 � 10 24−25 cm −2 , which correspond to more than 1000 magnitudes of extinction toward the nuclei at visual wavelengths. Finally, the molecular gas mass densities and line-of-sight velocity dispersions show significant overlap with stellar densities and line-of-sight stellar velocity dispersions of local elliptical galaxies with MV < 19 mag, including rapidly rotating ellipticals with disky isophotes and power-law light profiles as well as slowly rotating ellipticals with boxy isophotes and cores. This provides strong evidence that the CO-rich nuclei of these ULIGs have the phase-space density of gas necessary to form the stellar cores of elliptical galaxies. Subject headings: galaxies: active — galaxies: interacting — galaxies: ISM — galaxies: individual — ISM: molecules — infrared: galaxies –

MOLECULAR GAS AND NUCLEAR ACTIVITY IN RADIO GALAXIES DETECTED BY IRAS

This paper reports the latest results from a millimeter-wave (CO) spectroscopic survey of IRAS-detected radio galaxies with L1.4 GHz ~ 1023-1028 W Hz-1 in the redshift range z ~ 0.02-0.15. The IRAS flux-limited sample contains 33 radio galaxies with different radio morphologies and a broad range of infrared luminosities (LIR = 109-1012 L?), allowing for an investigation of (1) whether low-z radio-selected active galactic nuclei (AGNs) reside in molecular gas-rich host galaxies and (2) whether the CO properties are correlated with the properties of the host galaxy or the AGN. All of the radio galaxies in Mazzarella et al. and Mirabel et al. have been reobserved. Three new CO detections have been made, raising the total number of CO detections to nine and setting the survey detection rate at ~25%. Many of the CO lines have double-peaked profiles, and the CO line widths are broad (average ?vFWHM ~ 500 ? 130 km s-1), exceeding the average CO widths of both ultraluminous infrared galaxies (300 ? 90 km s-1) and Palomar-Green QSOs (260 ? 160 km s-1), and thus being indicative of massive host galaxies. The CO luminosities translate into molecular gas masses of ~(0.4-7) ? 109 M?, however, the 3 ? CO upper limits for nondetections do not rule out a molecular gas mass as high as that of the Milky Way (~3 ? 109 M?). Optical images of eight out of nine molecular gas-rich radio galaxies show evidence of close companions and/or tidal features. Finally, there is no obvious correlation between radio power and molecular gas mass. However, it is notable that only one F-R II galaxy out of 12 is detected in this CO survey; the remaining detections are of galaxies hosting F-R I and compact radio jets.

Molecular Gas in Infrared-Excess, Optically Selected and the Quasars Connection with Infrared-Luminous Galaxies

The initial results of a millimeter (CO) survey of infrared-excess, optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey with redshifts in the range 0.04 0.36, in which the contribution to the bolometric luminosity of infrared thermal dust emission for all PG QSOs is typically 20%–40%. Six out of ten of the QSOs observed are detected in the CO (1 → 0) emission line; two detections confirm previous, less sensitive detections of CO (1 → 0) in PG 1613+658 and 0838+770, and four additional QSOs are detected for the first time (PG 1119+120, 1351+640, 1415+451, and 1440+356). These six detections, plus two previous detections of CO in I Zw1 and Mrk 1014, bring the total number of 0.04 < z < 0.17 infrared-excess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling star formation, molecular gas may also serve as a primary source of fuel for QSO activity. Both the eight QSOs detected in CO and the four QSOs with nondetections have high infrared-to-CO luminosity ratios, LIR/L, relative to most infrared luminous galaxies of the same LIR. The placement of these QSOs on the LIR/L–LIR plane may be due to significant contributions from dust heated by the QSO in their host galaxies, due to dust heated by massive stars formed with high efficiency (i.e., per unit molecular gas mass) relative to most infrared luminous galaxies, or a combination of both. If the observed high values of LIR/L are primarily due to dust heating by QSOs, a significant fraction of ULIGs with similar values of LIR/L may also contain buried active galactic nuclei. Alternatively, if high LIR/L is due primarily to star formation, then an enhanced star formation rate may be intimately connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetected QSOs with the optical morphologies of their host galaxies shows that the three QSOs with LIR and L similar to ULIGs appear to reside in morphologically disturbed galaxies (i.e., ongoing major mergers involving two or more gas-rich disk galaxies), whereas the host galaxies of the remaining eight QSOs with lower LIR and L appear to be a mixture of barred spiral host galaxies, elliptical galaxies, galaxies with an indeterminate classification, and at least one ongoing major merger.

DENSE MOLECULAR GAS AND THE ROLE OF STAR FORMATION IN THE HOST GALAXIES OF QUASI-STELLAR OBJECTS

New millimeter-wave CO and HCN observations of the host galaxies of infrared-excess Palomar-Green (PG) quasi-stellar objects (QSOs) previously detected in CO are presented. These observations are designed to assess the validity of using the infrared luminosity to estimate star formation rates of luminous active galactic nuclei (AGNs) by determining the relative significance of dust heating by young, massive stars and AGNs in QSO hosts and IRAS galaxies with warm, AGN-like infrared colors. The analysis of these data is based, in part, on evidence that HCN traces high-density (>104 cm-3) molecular gas, and that the starburst-to-HCN luminosity ratio, LSB/L, of IRAS-detected galaxies is constant. The new CO data provide a confirmation of prior claims that PG QSO hosts have high infrared-to-CO luminosity ratios, LIR/L, relative to IRAS galaxies of comparable LIR. Such high LIR/L ratios may be due to significant heating of dust by the QSO or to an increased star formation efficiency in QSO hosts relative to the bulk of the luminous IRAS galaxy population. The HCN data show a similar trend, with the PG QSO host I Zw 1 and most of the warm IRAS galaxies having high LIR/L (>1600) relative to the cool IRAS galaxy population, for which the median LIR/Lcool ~ 890. If the assumption is made that the infrared emission from cool IRAS galaxies is reprocessed light from embedded star-forming regions, then high values of LIR/L are likely the result of dust heating by the AGNs. Further, if the median ratio of L/L ~ 0.06 observed for Seyfert galaxies and I Zw 1 is applied to the PG QSOs not detected in HCN, then the derived LIR/L values correspond to a stellar contribution to the production of LIR of ~7%-39%, and star formation rates of ~2-37 M⊙ yr-1 are derived for the QSO hosts. The corresponding values for the warm galaxies are ~10%-100% and ~3-220 M⊙ yr-1. Alternatively, if the far-infrared is adopted as the star formation component of the total infrared in cool galaxies, i.e., LFIR/Lcool ~ LSB/L, the stellar contributions in QSO hosts and warm galaxies to their LFIR are up to 35% and 10% higher, respectively, than the percentages derived for LIR. This raises the possibility that the LFIR in several of the PG QSO hosts, including I Zw 1, could be due entirely to dust heated by young, massive stars. Finally, there is no evidence that the global HCN emission is enhanced relative to CO in galaxies hosting luminous AGNs.

Near-Infrared Spectroscopy and a Search for CO Emission in Three Extremely Luminous IRAS Sources: IRAS F09105+4108, IRAS F15307+3252, and PG 1634+706

Rest-frame 0.48-1.1 μm emission-line strengths and molecular gas mass (H2) upper limits for three luminous infrared sources—the hyperluminous infrared galaxies IRAS F09105+4108 (z = 0.4417), IRAS F15307+3252 (z = 0.926), and the optically selected QSO PG 1634+706 (z = 1.338)—are presented. Diagnostic emission-line ratios ([O III] λ5007/Hβ, [S II] λλ6716, 6731/Hα, [N II] λ6583/Hα, and [S III] λλ9069, 9532/Hα) indicate a Seyfert 2-like spectrum for both infrared galaxies, consistent with previously published work. The upper limits of molecular gas mass for all three sources are M(H2) < (1-3) × 1010 h-2 M☉ (q0 = 0.5, H0 = 100 h km s-1 Mpc-1), less than that of the most gas-rich infrared galaxies in the local universe. All three sources have L/L~1300-2000 and thus are the extragalactic sources with the most extreme L/L values measured to date. Given the relatively warm far-infrared colors for all three objects, much of their infrared luminosity may emanate from a relatively modest amount of warm dust (e.g., Md ~ 105-107 h-2 M☉, Td = 200-100 K) near the AGN. For F09105+4108 and F15307+3252, the implied circumnuclear covering factor of this dust is ~90%, while for PG 1634+706 the covering factor is only ~35%.